This invention relates to a method for preparing a gallium nitride phosphor, and more particularly to a method for preparing a Ga.sub.1-x In.sub.x N:A,B (0.ltoreq.x&lt;1, A=Zn,Mg, B=Si,Ge) phosphor.
In recent years, it has been known that gallium nitride (hereinafter generally referred also to as "GaN") emits blue and green light at high luminance when it is incorporated in the form of a single crystal in a luminescent device such as an LED, an LD or the like. Also, it emits light in a wide luminous color range extending from a blue luminous color to a red luminous color when it is represented by a general formula Ga.sub.1-x In.sub.x N:A,B (0.ltoreq.x&lt;1, A=Zn,Mg, B=Si,Ge)
Preparation of a GaN phosphor which has been conventionally known in the art is carried out by blending a Ga compound acting as a starting material with a compound for a dopant (dopant compound) to prepare a blend and then placing the blend in a calcination oven to subject it to calcination at a high temperature while exposing it to ammonia flowed in the oven, to thereby nitride Ga and dope it with the dopant.
In the past, it has been attempted to carry out luminescence of the thus-prepared GaN phosphor by means of electron beams. However, the prior art fails to permit the phosphor to emit light at increased luminance when it is in the form of a powder.
The major reason why the phosphor fails to exhibit increased luminance is that nitriding of the GaN phosphor is hard, unlike other phosphors. More particularly, the GaN phosphor causes a difference between a temperature at which it is nitrided (700 to 1000.degree. C.) and a temperature at which it starts to be composed (950.degree. C.) to be reduced, so that normal heating of the phosphor for reaction thereof tends to cause nitriding and decomposition of the phosphor to concurrently progress. Thus, although the reaction leads to formation of GaN, the GaN obtained fails to exhibit crystallizability sufficient to permit it to satisfactorily act as a phosphor intended.
Also, the GaN phosphor exhibits pair-luminescence by a combination of a donor increased in valence by one as compared with Ga and an acceptor decreased in valence by one as compared with Ga, so that it is required to dope GaN with a dopant for each of the donor and acceptor. Doping of the dopant requires a significantly increased temperature. However, GaN tends to be decomposed at such a high temperature, resulting in failing in an increase in temperature, leading to a failure in satisfactory doping.
Further, preparation of a nitride such as GaN is generally carried out by subjecting a starting material or Ga compound to calcination at a high temperature in an atmosphere using ammonia, during which hydrogen produced by decomposition of ammonia exhibits a reducing action to a high degree. The reducing action causes GaN to be reduced, resulting in Ga being liberated from GaN, leading to blackening of the phosphor. Such blackening of the phosphor gives rise to a fatal disadvantage because it causes the phosphor to absorb its own luminescence, resulting in the phosphor being reduced in luminance. Also, the reducing action causes decomposition and scattering of the dopant incorporated in the GaN, leading to a failure in satisfactory diffusion of the dopant. In order to eliminate such disadvantages due to the reducing action of hydrogen, it is required to carry out the reaction at a low temperature. However, this fails to dope GaN with the dopant. Thus, the prior art fails to provide a GaN phosphor which is capable of emitting light at increased luminance.